The measurement of mean and of fluctuating wall shear-stress in laminar, transitional, and turbulent boundary layers and channel flows has been used in industry and the scientific community. Measurement of mean shear-stress is related to the global state of fluid flow and may be used to determine the viscous skin-friction drag caused by the fluid on a body. The time-resolved, fluctuating shear-stress is a footprint of the turbulent processes responsible for the unsteady transfer of momentum to a body. Haritonidis, J. H. “The Measurement of Wall Shear Stress,” Advances in Fluid Mechanics Measurements, Ed. by M. Gad-EI-Hak, Springer-Verlag, 1989, pp. 229–261. Fluctuating shear-stress data can also provide physical insight into complex flow phenomena, including turbulent viscous drag, transition to turbulent flow, flow separation, and shock-wave/boundary layer interactions.
Accurate measurement of skin friction is of vital importance in numerous industries. For instance, skin friction drag forms approximately 50 percent of total vehicle drag for a typical subsonic transport aircraft. Hefner, J. N. and Bushnell, D. M., “An Overview of Concepts for Aircraft Drag Reduction,” AGARD-R-654, 1977. As a result, accurate design of airfoils and other bodies requires accurate estimates of skin friction drag. Accurate measurement of wall shear-stress is vital for turbulence modeling and simulation validation as well as for the accurate assessment of skin friction drag reduction concepts. In supersonic flows, the measurement of wall shear-stress is critical to the understanding of shock-wave/boundary layer interactions which directly influence critical vehicle characteristics such as lift, drag, and propulsion efficiency. Gaitonde, D., Knight et al., “White Paper: Shock-Wave/Boundary Layer Interaction Research,” AFOSR workshop on Shock-Wave/Boundary Layer Interactions organized by J. D. Schmisseur, May, 2002. In non-aerospace or hydrodynamic applications, the measurement of shear-stress can be used for industrial process control. Goldberg, H. D., Breuer, K. S. and Schmidt, M. A., “A Silicon Wafer-Bonding Technology for Microfabricated Shear-Stress Sensors with Backside Contacts,” Technical Digest, Solid-State Sensor and Actuator Workshop, 1994, pp. 111–115. In biomedical applications, both mean and fluctuating wall shear-stress are important hemodynamic factors in the development of arterial pathologies, such as atherosclerosis. Grigoni, M., Daniele, C., D'Avenio, G. and Pontrelli, G. “The Role of Wall Shear Stress in Unsteady Vascular Dynamics,” Progress in Biomedical Research, Vol. 7, No. 3, 2002, pp. 204–212.
Many different devices have been used to attempt to accurately determine shear-stress at walls susceptible to shear-stress from fluids flowing past the walls. For instance, clauser-plot techniques, preston tubes, obstacle methods, hot-film anemometers, mass-transfer probes, oil-film techniques, and liquid crystal methods have all been used; however, each with limited success. Winter, K. G., “An Outline of the Techniques Available for the Measurement of Skin Friction in Turbulent Boundary Layers,” Progress in the Aeronautical Sciences, Vol. 18, 1977, pp. 1–57. It has been reported that uncertainties for mean shear-stress for surface fence methods, wall hot wires, wall pulsed wires, and oil-film are about 5 percent in incompressible flows and about 10 percent for supersonic flows. Naughton, J. W. and Sheplak, M. “Modern Developments in Shear Stress Measurement,” Progress in Aerospace Sciences, Vol. 38, 2002, pp. 515–570. Accurate, direct measurement of fluctuating wall shear-stress has not been realized using conventional technologies.
Microelectromechanical systems (MEMS) are devices that operate on a very small scale, typically in a range of tens of microns to a few millimeters, and have been used to form shear-stress sensors. In some applications MEMS devices are imperceptible to the unaided human eye. MEMS devices mostly are fabricated using integrated circuits (IC) technology. MEMS devices include many different devices used for a variety of purposes. For instance, MEMS technology has been used to create shear-stress sensors; however, some MEMS shear-stress sensors have not achieved a desired level of performance of bandwidth, spatial resolution, stability, integration range, etcetera. For instance, thermal MEMS sensors have often not been accurate because of difficulty in obtaining unique calibration between heat transfer and wall shear-stress, measurement errors associated with mean temperature drift, and flow perturbations due to heat transfer to the flow.
In addition to these shear-stress sensing devices, MEMS floating element shear-stress sensors having been developed; however, the performance of these devices have suffered as well. For instance, a MEMS floating element sensor has been produced using a polyimide/aluminum surface micromachining process; however, the device was susceptible to moisture, which caused the mechanical properties of the device to change and caused mechanical sensitivity drift due to induced swelling. Schmidt, M. A., Howe, R. T., Senturia, S. D., and Haritonidis, J. H. “Design and Calibration of a Microfabricated Floating-Element Shear-stress Sensor,” Transactions of Electron Devices, Vol. ED-35, 1988, pp. 750–757. In addition, air-dielectric interfaces subjected to charged species accumulation appeared as drift when detected by capacitive plates. Naughton, J. W. and Sheplak, M. “Modern Developments in Shear Stress Measurement,” Progress in Aerospace Sciences, Vol. 38, 2002, pp. 519. Another floating element shear-stress sensor employed differential optical-shutter-based floating element sensors for turbulence measurements; however, the performance of this sensor suffers from front-side electrical contacts that interfere with fluid flow past the sensor and from remote mounting of the incident light source. Padmanabhan, A., Sheplak, M., Breuer, K. S., and Schmidt, M. A., “Micromachined Sensors for Static and Dynamic Sheer Stress Measurements in Aerodynamic Flows,” Proc. Transducers 97, Chicago, Ill., 1997, pp. 137–140. A floating element shear-stress sensor employs a capacitive sensing scheme. In general, capacitive shear-stress sensors do not possess favorable scaling with shrinking size. Gabrielson, T., B., “Mechanical-thermal Noise in micromachined Acoustic and Vibration Sensors,” IEEE Electron Devices, 40, 1993, pp. 903–909. Specifically, the electrical sensitivity is directly proportional to the electrode surface area, while the thermodynamic minimum detectable signal is inversely proportional to area. Gabrielson, 1993.
Other floating element shear-stress sensors have been developed; however, the performance of each device suffers as well. Thus, a need exists for a more accurate floating element shear-stress sensor.